Jim McGrain, DVM

This time of year, with the frequency of round bales being fed, we tend to receive questions regarding the disease Botulism. Botulism is a bacteria that, under anaerobic (absence of oxygen) conditions, produces spores that, in turn, release a toxin. There are several distinct Botulism toxins ranging from A – H with types B and C being of primary concern in the horse. Type B is usually the result of a contaminated feed material containing decaying forage matter. This is why we can be concerned about round bales, particularly the center where anaerobic, moist conditions may exist. The type C toxin
originates from the decomposition of contaminated animal carcasses in the feed material.

Vaccination for Botulism is available, but will only protect against the Type B toxin.,br>
Clinical signs of Botulism in the horse usually occur within 12 – 24 hours after ingestion. Progression of the disease varies based on amount of spore or toxin ingested. Initially, horses may appear weak, lethargic, run a low-grade fever (101 – 102), act mildly colicky, and/or be very slow to eat. With the toxin affecting the motor nerves, neurologic signs will begin to become more pronounced. Flaccid paralysis usually begins with the face so that the eyelids become droopy and the tongue has difficulty retracting back into the mouth. These signs can progress to weak tail tone and a weak hind end. Horses begin to spend more time down requiring an increased effort to get up until eventually they become completely recumbent. The end result can cause paralysis of the diaphram and death.

Treatment is usually expensive, intensive, and carries a guarded prognosis. The earlier the disease is recognized and treatment with antiserum is initiated, the better the outcome is for the horse. Once the horse progresses to complete recumbency, the disease is fatal.
Prevention is to purchase round bales that have been stored under ideal conditions and inspecting the round bales, especially as the horses get into the center, for spoiled forage.

There is a vaccination available that is safe and effective against the type B toxin. Three initial doses every 3-4 weeks are given, followed by an annual booster. Anyone feeding round bales should consider this preventative measure.

Please contact us if you have any questions regarding vaccinating your horse.

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